In breaking waves, whitewater, or ships’ wakes, water currents pull foaming bubbles beneath the surface. Seen through the water, the foam appears warmer and darker. As it rises to the surface, it forms ringlets of bubbles around each spreading cell of rising water.
Above: William Trost Richards, “The Storm” (click to enlarge).
The bubbles quickly disperse as the upwelling cell of water travels outward.
To make the foam appear to ride on the surface of the water, it should be painted last, while the underwater foam should be painted first. Above: A detail of an oceanscape by Montague Dawson (1895-1973). Full image at Animation Treasures